1. Field of the Invention
The present invention relates to a toner used for developing an electrostatic latent image formed by an electrophotographic method, an electrostatic recording method or the like, and more particularly concerns a color toner that is suitable for a non-contact heat fixing system.
2. Description of the Background Art
With respect to the method for heat-fixing a toner image on copying paper, there are basically two methods, that is, a contact heat fixing system and a non-contact heat fixing system. The non-contact heat fixing system is a fixing system in which upon fixing, no members contact an image made from toner powder, and mainly classified into a flash fixing system and an oven (atmosphere) fixing system.
In the flash fixing system, a powder toner image, transferred onto copying paper from a photosensitive member or an intermediate transferring member, is irradiated with flash light from, for example, a xenon lamp, a halogen flash lamp or the like so that the toner image is fused by the radiation heat, and fixed onto the copying paper.
In the oven fixing system, a fine particle toner image, transferred onto copying paper from a photosensitive member or an intermediate transferring member, is irradiated with infrared rays under an oven atmosphere so that the toner image is fused by the radiation heat, and fixed onto the copying paper.
These non-contact heat fixing systems have the following superior features.    Since the powder toner image is fused and fixed-without contacting any members, the toner image is free from damages caused by those members so that upon developing, there is no degradation in the resolution.    Since the fixing time is very short, a high-speed fixing process is available.    Since no waiting time is required for the fixing process, it is possible to start the process quickly.    These systems are readily applied to various kinds of copying paper having different thicknesses and qualities.
However, since the non-contact heat fixing system carries out a heat fixing process in a non-contact state, its ambient energy dissipation is great. From the environmental viewpoint, there have been demands for a reduction in fixing energy. Even under such circumstances, when the total amount of light energy to reach the powder toner image is insufficient, the powder toner image is not sufficiently fused, resulting in the problem of insufficient fixing characteristics. In particular, in the case of a full-color image in which black-color images and color images are simultaneously printed, since the amount of energy to be absorbed is different depending on the respective colors, it is very difficult to control the amount of energy to be applied.
For this reason, in order to achieve a sufficient melt-fixing process, for example, a flash fixing toner having a specific loss elastic modulus at a specific temperature has been proposed (for example JP-A-11-184142 (claim 1)).
However, the above-mentioned toner has failed to provide sufficient color fixing properties depending on types and fixing conditions of images. For example, in the case when the fixing energy is comparatively small, even if desired color reproducibility is obtained in the case of a mono-color image which is formed by a single color, there is degradation in the color reproducibility (degradation in the color-mixing property) when an image having two superposed colors or three superposed colors, such as a full-color image, is fixed. This causes the disadvantage of degradation in smear-preventive property. In other words, in the case when the fixed image is used for a label of a bottle or the like, the image tends to have degradation in the image quality, such as blurring or stains, when rubbed against a packing member or the like, or tends to be transferred onto another member. In the case when copy paper bearing an image formed on at least one surface thereof is fed, the fixed image tends to be rubbed against a roller or the like to cause degradation in the image quality such as blurring or stains; this causes another disadvantage of degradation in smear-preventive property. These problems, in particular, the problem of degradation in the color reproducibility (degradation in the color-mixing property), become more conspicuous as the system speed of an image-forming apparatus is increased.
In order to solve such problems, in particular, the problem of degradation in the color reproducibility, in the case when the fixing energy is increased, although desired image quality is obtained in the case of a mono-color image, a white blank phenomenon occurs upon fixing an image with two superposed colors or three superposed colors having comparatively great amount of toner adhesion, causing image-density irregularities and irregularities in gloss. The white blank phenomenon is an inherent phenomenon in the non-contact heat fixing system in which one portion of a color image appears to be a white blank having a round shape or an elliptical shape. It is considered that the white blank phenomenon is caused by the fact that, when a powder toner image is instantaneously fused excessively, air located between the toner particles appears to the surface of the toner image in a certain collected amount (bumping phenomenon). When the amount of fixing energy is increased, the energy absorption becomes too high in black toner portions to cause a bumping phenomenon in the same manner as the full-color image, resulting in image-density irregularities and irregularities in gloss.